Method for Data Transmission in an Automation System Using Dynamic Frame Packing

ABSTRACT

A method for data transmission in an automation system from a second field device via a first field device to a receiver, wherein, at the first field device, a first data subframe is created, a second data frame is received from the second field device, and a first data frame including the first and the second data subframe is sent correctly-timed by Dynamic Frame Packing to the receiver. In the event the second data subframe is unable to be appended directly to the first data subframe at the latest after the correctly-timed sending of the first data subframe, the sending of the first data frame is shifted by a time value, where the time value is calculated so that immediately after the sending of the first data subframe, the second data subframe is able to be appended directly to the first data subframe.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to data communications and, more particularly, toa method for automated data transmission in automation system, acomputer program product and to a field device for data transmission inan automation system using Dynamic Frame Packing.

2. Description of the Related Art

The known concept of Dynamic Frame Packing (DFP) was introduced toincrease the transmission speed of data from field devices tocontrollers in an automation system. In DFP, data is transmitted usingcontainer frames. Terminals that are assigned to a packing group thentransmit the data within these container frames. The advantage oftransmitting data in this manner is that the overhead of Ethernet framesused for a given transmission is only counted once, since, because ofthe container frame, for example, only a preamble, a start framedelimiter and a header are used. As a result, it is possible to increasethe packing density so that, within one clock cycle, data can betransmitted by a plurality of field devices, with the updating rates inrelation to the transmission being significantly increased in comparisonto data transmission that would not use DFP.

FIG. 1 shows the structure of a conventional real-time frame, with thisframe itself being encapsulated for Ethernet transmissions in accordancewith Request for Comment (RFC) 894 using the elements destinationaddress, source address, Ethertype and cyclic redundancy check (CRC). Inthe event of priority tagging being used in accordance with Institute ofElectrical and Electronic Engineers (IEEE) standard 802.1Q with theEthertype set to 0x8100, followed by a priority/VLAN field, followed bya second Ethertype, which is set to 0x8892 and which indicates areal-time frame.

The frame ID is used to identify the frame itself, whereby the C_SDU isintended to transport the IO data of the field devices and the APDUstatus specifies the status of the frame.

In these cases, the C_SDU can be structured so that it either carriesthe IO data of an individual field device or so that it carries the IOdata of a number of field devices. In the latter case, where the C_SDUcarries a number of field devices, a part of the C_SDU (i.e., thesubframe) carries the content of a specific field device. Here, theframe is subdivided into a number of subframes.

The reason for using a subdivision into frames is to minimize therequired bandwidth and to explicitly optimize the overall systemperformance. As shown in FIG. 1, the overhead of a frame, i.e., thebytes to be transported other than the C_SDU, amounts to 28 bytes.However, the InterFrameGap additionally amounts to 12 bytes. As aresult, the preamble amounts to 7 bytes and the start frame delimiteramounts to 1 byte and these also have to be taken into account, i.e.,the sum total of the overhead of a frame amounts to 48 bytes (or 42bytes if the preamble is shortened to 1 byte). If the C_SDU is alsosmaller than 40 bytes the difference must also be added in.

It is thus evident that the use of subdivided frames reduces therequired bandwidth by combination of subframes 110 and 112, since, asshown in FIG. 2, the overhead of a subframe merely amounts to 6 bytes.By combining a number of subframes into an individual frame, theoverhead for the frame that is used is only counted once.

Assigned to each of the subframes 110 and 112 is a position, a controlbit, a data length that describes the length of the C_SDU, a cyclecounter, a data status and a CRC. Here, the position involves a uniqueidentifier for a given subframe, whereby the list of subframes isterminated by a specific subframe with the position number 0.

The control bit serves to specify whether or not the CRC and cyclecounter of the subframe are to be ignored.

The data status of a subframe specifies the data status of the subframe.The data status within the APDU status of the frame specifies the datastatus of the frame. In the event of the frame consisting of subframes,the data status of the frame can be ignored. It is also helpful toassign a static value to the data status of the frame.

SUMMARY OF THE INVENTION

It is an object of the invention is to provide a method for datatransmission in an automation system, a computer program product and afield device for data transformation in an automation system.

This and other objects and advantages are achieved in accordance withthe invention by a method for data transmission in an automation systemfrom a second field device via a first field device to a receiver,wherein at the first field device, a first data subframe is created,where the first data subframe comprises process data of the first fielddevice; a second data frame is received from the second field device,where the second data frame comprises a second data subframe withprocess data of the second field device; and a first data framecomprising the first and the second data subframe is sentcorrectly-timed by Dynamic Frame Packing (DFP) to the receiver.

In this case, in the event that a second data subframe that is sent cannot be appended immediately to the first data subframe at the latestafter the correctly-timed sending of the first data subframe, thesending of the first data frame is shifted by a time value, where thetime value is calculated so that immediately after the first datasubframe is sent the second data subframe is able to be appendeddirectly to the first data subframe.

In accordance with the invention, the above described receiver can, forexample, comprises a controller of the automation system, i.e., an IOcontroller (IOC). Alternatively, the receiver can also comprise afurther field device that is connected downstream from the first fielddevice.

The disclosed embodiments of the invention have the advantage that,because of the calculation of an optimum transmission time of the firstdata frame with the seamless concatenation of the first and second datasubframes, real-time data transmission in the automation system isguaranteed.

Expressed in simple terms, the beginning of the transmission time of thefirst data frame is delayed until such time as the second data subframe,i.e., as soon as this is available at the first field device for DynamicFrame Packing, can be seamlessly added to the first data subframecontained in the first data frame. Thus, as soon as the second datasubframe is available for Dynamic Frame Packing at the first fielddevice, the second data subframe is added by adding it to the first datasubframe already in the process of being sent at this moment. The seconddata subframe is thus directly appended to the first data subframe bythe correctly-timed sending, with correctly-timed meaning that thesecond data subframe can be appended to the first data subframe at thecorrect time, i.e., there is no gap in the form of a wait time that hasto be filled by a “gap frame” after sending out the first data subframe.Here, the second data subframe does not yet have to be completelyreceived, i.e., the frame can be appended dynamically to the firstoutgoing data subframe even during the receiving (input) of the seconddata subframe.

In accordance with an embodiment of the invention, the transmission timeof the second data frame from the second to the first field device isincluded in the calculation of the time value. For example, thetransmission time includes a time delay for sending the second dataframe by the second field device, a time delay for receiving the seconddata frame by the first field device and/or a time delay through thecable delay time of the second data frame between the first and thesecond field devices.

This enables the extent to which the sending of the first data framemust be shifted in time to take into account the hardware circumstancesof the first and second field device to be established precisely. Inaddition, the spatial distance between the first and second fielddevices is taken into account by taking into account the cable delaytime, i.e., in the event of the first and second field devices being faraway from each other, a corresponding signal between the first andsecond field devices would need a longer period of time, starting fromthe second field device, in order to arrive at the first field device.All this is taken into account in an optimum manner by in accordancewith the disclosed embodiments of the method of the invention.

In accordance with a further embodiment of the invention, thecalculation of the time value includes the lead time for performing theDynamic Frame Packing at the first field device. This means that heretoo the hardware in the form of the first field device is taken intoaccount in the computation of the time value. The second data subframecan thus be appended to the first data subframe in an optimum manner.

In accordance with a further embodiment of the invention, the timeoffset of the local time systems in the first and second field device,i.e., the “Peer-to-Peer-Jitter”, is taken into account in thecalculation of the time value.

In accordance with a further embodiment of the invention, the first dataframe is sent via a further field device to the receiver, with datatransmission of all field devices occurring in a time-synchronizedmanner, where for all field devices a global and/or an individual localsend start offset is predetermined as time delay, where a beginning ofthe data transmission in a field device is delayed in relation to thebeginning of the data transmission in a neighboring field device atleast by the send start offset, and where the sending of the first dataframe by the first field device occurs at the earliest after the sendstart offset has elapsed. In other words, either in a global manner orindividually a suitable value of a time delay can be specified for eachfield device, in accordance with which the respective field devicebegins its send process in relation to a neighboring field device.

In accordance with a further embodiment of the invention, in the eventof the sum of transmission duration of the second data frame from thesecond to the first field device, the time offset of the local timesystem in the first and second field device and/or the lead time forperforming the Dynamic Frame Packing in the first field device beinggreater than the duration for sending the entire first data subframe,the time value becomes negative. In such cases, the duration for sendingthe entire first data subframe is governed by its size. Thus, the largerthe first data subframe, the more time will be needed to transferindividual bits of the data subframe sequentially to the receiver.

In accordance with a further embodiment of the invention, the time valueis calculated from the sum of transmission duration of the second datafrom the second to the first field device, time offset to local timesystem in the first and second field device, the lead time forperforming the Dynamic Frame Packing in the first field device and thetime value by which the sending of the second data frame at the secondfield device was delayed for the purposes of correctly-timed sending,minus the duration for sending the entire first data subframe. Here, thetime parameters of this sum can be included in their entirety in thecalculation of the time value. However, it is also possible for onlyparticular individual parameters to be included in the calculation ofthe time value. The inclusion of only particular individual parametersin the calculation of the time value can be especially useful if thesize of the individual parameters are particularly negligible.

In accordance with a further embodiment of the invention, in the eventof the calculated time value being smaller than the send start offset ofthe first field device, the send start offset of the first field deviceis used as the time value.

Another object of the invention is to provide a computer program productwith instructions able to be executed by a processor (ApplicationSpecification Integrated Circuit (ASIC)) for performing the method stepsas mentioned above.

Another object of the invention is to provide a first field device,where the first field device is configured to receive data from a secondfield device and is configured to send data to a receiver. The firstfield device is further configured to create a first data subframe,where the first data subframe comprises process data of the first fielddevice; receive a second data frame from the second field device, wherethe second data frame includes a second data subframe with process dataof the second field device; send correctly-timed to the receiver a firstdata frame comprising the first and the second data subframe by DynamicFrame Packing (DFP),

In the event that a second data subframe that is sent can not beappended immediately to the first data subframe at the latest after thecorrectly-timed sending of the first data subframe, the sending of thefirst data frame is shifted by a time value, where the time value iscalculated so that immediately after the first data subframe is sent thesecond data subframe is able to be appended directly to the first datasubframe.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not necessarily drawn to scale and that, unless otherwiseindicated, they are merely intended to conceptually illustrate thestructures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are explained in greater detailbelow with reference to the drawings, in which:

FIG. 1 shows the structure of a conventional real-time frame;

FIG. 2 shows the structure of a sub-divided conventional real-timeframe;

FIG. 3 shows the sequence of a method for data transmission usingdynamic frame packing (DFP) in accordance with the invention;

FIG. 4 shows the sequence of a method for data transmission usingdynamic frame packing (DFP) in accordance with an embodiment of theinvention;

FIG. 5 shows a schematic block diagram of the functioning of dynamicframe packing (DFP) in accordance with the invention;

FIG. 6 shows a schematic diagram of dynamic frame packing (DFP) withadaptation of the size of data subframes in accordance with theinvention; and

FIG. 7 shows a schematic diagram of the execution of dynamic framepacking (DFP) using correctly-timed sending in accordance with theinvention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Elements similar to one another are identified below by the samereference characters.

FIG. 3 shows a method for data transmission using DFP in accordance withthe invention. Here, it is assumed in the following explanation that, inan automation system, as well as a controller (IOC) 100, four fielddevices (10 devices) 102, 104, 106 and 108 are arranged topologicallybehind one another. Data transmission using Dynamic Frame Packing occursin this method, starting from IOC 100, to the field devices 102, 104,106 and 108 so that the IOC 100 transfers a packet to the field device102, with this packet comprising a number of subframes 110, 112, 114 and116, which each contain user data for the field devices 102, 104, 106 or108. The container 150 used for data transmission, in addition to thesubframes 110, 112, 114, 116 also has an Ethernet Header 120 and anEthernet Trailer 118.

After receipt of the container 150 by the field device 102, the fielddevice 102 extracts the data subframe 110 intended for it from thecontainer 150 and then sends, in the step labeled with the referencecharacter 130, a new packet to the field device 104. This new packet,however, now no longer contains the data subframe 110 but only the datasubframes 112, 114 and 116, which are intended for the topologicallydownstream field devices 104, 106, 108. The Ethernet Header 120 istransferred unchanged while a new Ethernet Trailer 122 is created and isappended to the data packet.

After receipt of the data packet created in this way by the field device104, the field device 104 extracts the data subframe 112 intended for itand creates in step 132 in its turn a new packet, where this new packetnow only comprises the data subframes 114 and 116, as well as theEthernet Header 120. The packet created in this way also comprises a newEthernet Trailer 124.

The packet created in this way is then in its turn received by fielddevice 106, which removes the data subframe 114 and in step 134transfers the remaining data subframe 116 together with the EthernetHeader 120 and a new Ethernet Trailer 126 to the last field device 108.

It can thus be stated in summary that each IO device (each field device102, 104, 106 and 108) in a consecutive sequence, starting fromcontroller 100, is supplied with frames, where after each receipt of thepacket, the respective field device forwards the packet in modified formto the downstream field device.

FIG. 4 shows the sequence of a method for data transmission using DFP inaccordance with an alternative embodiment of invention, Here, however,by contrast to the diagram in FIG. 3, a data transmission is initiatedby the field device 108. The field device 108 sends a frame withEthernet Header and Ethernet Trailer as well as a data subframe 116embedded therebetween to the field device 106, which then appends itsdata subframe 114 to the data subframe 116. The data packet created inthis way is transferred to the field device 104, which in its turn addsits data subframe 112 to the packet and transfers the frame created inthis way to the field device 102. The field device 102 appends its datasubframe 110 to the data subframe 112 of the field device 104 andforwards the frame created in this way to the controller 100.

FIG. 5 shows schematically how Dynamic Frame Packing is performed asregards timing. In this figure, the abscissa represents the distancecovered by data packets, while the time needed for transmission isplotted on the ordinate.

In this case, the schematic representation of Dynamic Frame Packing(DFP) in FIG. 5 corresponds to the representation of DFP datatransmission in FIG. 4. Here, FIG. 5 shows ideal conditions for DynamicFrame Packing, i.e., the data subframes, starting from the right-handfield device through to the controller, arrive at the subsequent fielddevices with such correct timing that the data subframe received in thisway can be added readily and without any gap to the data subframe ofthis respective field device. Thus, for example, in FIG. 5 the datasubframe 116 reaches the field device 106 before this has completelysent the data subframe 114.

In the event of the field device 106 sending out the data subframe 114with a time delay, from this delay value (FSO, FrameSendOffset) a localperiod of time can be defined from which it can be concluded when thesending of the data subframe 114 is completed. Here, this time isproduced by the size of the data subframe 114, i.e., from the ratiobetween the first byte after the checksum of the data subframe and thefirst byte of the actual frame used for sending, i.e., the first byte ofthe destination address, multiplied by a value defined by the system,such as 80 ns. This produces the last point in time at which the datasubframe 116 must be available at the field device 106 for the purposesof DFP. Otherwise, a “DFP frame late error”, i.e. an error, would beproduced.

It should be noted that in the present case the Ethernet Header cansimply be ignored because this is transferred both by the field device108 and also by the field device 106. An FSO is also basically relatedto the start frame delimiter. Consequently, it is irrelevant whether ashort or a long preamble is used at a specific receive port of the fielddevice 106 (Tx port).

For this reason the following inequality applies:

FSO_(IODn)+MaxLineRxDelay+MaxDFP_Feed≦FSO_(IODn-1)+SubframeSize_(IODn-1)−PeerToPeerJitter

The left-hand side of this inequality describes that the data subframe nis transmitted by the field device n (IODn) at point in time FSO_(IODn),where the data subframe n must pass through the signal line betweenfield device n and n−1 (IODn and IODn−1). For this reason, aMaxLineRxDelay, i.e., the period of the transmission time of the dataframe between the field devices must be added. In this case, thistransmission time, in addition to a time delay caused by the cable delaytime of data frames between the field devices, can also include a timedelay (Tx Delay) on transmission of the data frame by a field device102, 104, 106, 108 and/or a time delay (Rx Delay) on receipt of the dataframe by a field device.

MaxDFP_Feed describes the lead time for performing the Dynamic FramePacking at the receiving field device 102, 104, 106, 108. MaxDFP_Feeddescribes the lead time that the field device 102, 104, 106, 108 needsto still be able to append the data of an incoming data subframe to thecurrently transmitted data frame.

The right-hand side of the above-mentioned inequality describes that thedata subframe n−1 is sent at point in time FSO_(IODn-1), where theduration SubframeSize_(IODn-1) is inserted as a period, i.e., the periodthat is needed for sending the subframe n−1. The FSO is either definedfor IODn−1 or IOD in the respective local time systems, which issynchronized with a SyncMaster. As a result, a PeertoPeer Jitter, i.e.,a timing offset to local time systems in the field devices is taken intoaccount. It is preferable for a packet with data subframes not to beallowed to arrive too late at the neighboring field device 102, 104,106, 108 to enable it to be appended in good time. Consequently, thisPeertoPeer Jitter is subtracted from FSO_(IODn-1).

The above-mentioned inequality can now be transformed into:

MaxDFP_Feed+MaxLineRxDelay+PeerToPeerJitter−SubframeSize_(IODn-1)≦FSO_(IODn-z)-FSO_(IODn)

MaxDFP_Feed, PeertoPeer Jitter, TxDelay and RxDelay (both components ofMaxLineRxDelay) are device properties which are predetermined by themanufacturer of the field device 102, 104, 106, 108. Only the cablelength, i.e., the signal delay time caused by said length, which is partof MaxLineRxDelay, can be configured by the user.

In order to now satisfy this inequality, the following conventionalprocess is implemented.

Within a DFP packing group essentially all field devices 102, 104, 106,108 begin the send process of their data subframes at the same point intime. In other words, the FSO amounts to an identical value for allfield devices 102, 104, 106, 108, so that FSO_(n-1)-FSO_(n) amount tozero. This results in:

MaxDFP_Feed+MaxLineRxDelay+PeerToPeerJitter−SubframeSize_(IODn-1)≦0

And from the foregoing, the following relationship is obtained:

${SubframeSize} \geq \frac{\begin{matrix}{{MaxDFP\_ Feed} + \begin{pmatrix}{{TxPortDelay} + {CableDelay} +} \\{RxPortDelay}\end{pmatrix}} \\{+ {PeerToPeerJitter}}\end{matrix}}{80\mspace{14mu} {ns}}$

The meaning of this is as follows: In the event of the actual real datasubframe size at a field device 102, 104, 106, 108 lying below thisorder of magnitude calculated in this formula, the corresponding size ofthe data subframe must be artificially increased. This is shown by wayof example in FIG. 6.

In FIG. 6 all field devices 102, 104, 106, 108 are connected one afterthe other and have the same FSO value, i.e., the same point in time atwhich a data transmission to a neighboring field device is begun. In anyevent, the problem arising in FIG. 6 is that the data subframe 116begins to be received by the field device 106 significantly later thanthe field device 106 has finished sending its data subframe 114. Inother words, the field device 106 finishes sending out the data subframe114 in the direction of the field device 104 before even starting toreceive the data subframe 116 from field device 108. In order, despitethis, not to have to forward the data subframe 116 in a separate frameto the field device 104, the field device 106 then introduces, inaccordance with the above described “SubframeSize” equation, a gapfiller 600, which thus follows immediately after the data subframe 114as data subframe 600. As soon as the data subframe 116 is received atthe field device 106, this will be dynamically appended to the datasubframe 600 being sent out at that moment, so that overall the frameshown in FIG. 6, consisting of header, data subframe 114, gap 600, datasubframe 116 and footer is produced.

However, the consequence of this is that in a clearly visible manner thesize of the frame sent in this way increases. This increases the amountof data to be transmitted and is therefore not so performant.

For this reason, the present invention proposes the method outlined inreference to FIG. 7, which makes real-time data transmission possiblebut, however, does not increase the data transmission overhead needed.In particular, instead of inserting a gap 600 after the data subframe114, the alternative embodiment of the invention provides a delay of thesend time of the frame at the field device 106 by a period 704 longenough for the data subframe 116 to be added seamlessly to the datasubframe 114. This produces the frame outlined in FIG. 7 in relation tothe field device 106, which consists of header 700, data subframe 114,data subframe 116 and footer 702.

In other words, the send start time FSO of the field device IOD_(n-1)(exemplary field device 106) is defined so that the data subframe 114sent out by this field device 106 ends precisely when the data subframeof the field device IOD_(n) (i.e., field device 108) is ready at thefield device 106 for performing DFP. The data subframe 116 is thus addeddirectly to the data subframe 114 without any gap. In an embodiment ofthe invention, it is possible for a global and/or an individual localsend start offset to be predetermined as a time delay. Such a send startoffset indicates that a start of data transmission in a field device102, 104, 106, 108 is delayed in relation to the start of the datatransmission in a neighboring field device 102, 104, 106, 108 at leastby this send start offset, in order to have the transmission alwaysoccurring in the network in a time section reserved for real-timecommunication. If, for example, a send start offset valid for all fielddevices (Global Start Offset) is provided, a respective send time at thefield devices 102, 104, 106, 108 should not occur before this send startoffset period has elapsed. This can be expressed by the followingformula:

FSO_(IODn-1)=MAX(FSO_(IODn)+MaxDFP_Feed+MaxLineRxDelay+PeerToPeerJitter−SubframeSize_(IODn-1);GlobalStartOffset)

In the specific case that the size of the data subframe of IOD_(n-1) isgreater than the sum of MaxDFP_Feed, MaxLineRXDelay and PeertoPeerJitter, the FSO of IODn−1 should be the same or brought forward inrelation to the FSO at IOD_(n). In other words, in this special case, inwhich the sum of transmission duration, time offset of the local timesystems and the lead time to perform the Dynamic Frame Packing isgreater than the time for sending the respective data subframe, the timeoffset 704 used is negative.

In summary, this produces the option of calculating an optimum send timefor data subframes to be sent. No gap frames are additionally appendedto data subframes, so that the overall size of the sent frames can bekept as small as possible.

A chain of ET 200 ecoPN 8DI, of a block periphery in IP67 with PROFINETconnection, which each need 8 bytes of subframe size, should bementioned here as a practical example.

If the known times for ERTEC200+ for MaxDFP_Feed and MaxLineRxDelay areused and if a cable length of 100 m is assumed, a minimum subframe sizeof 15 bytes is needed to implement the process discussed in FIG. 6, ifPeerToPeerJitter of 100 ns can be achieved. If the PeerToPeerJitter isgreater (e.g., 300 ns), only a minimum subframe size of 18 bytes isachievable.

Thus, in an implementation of the process discussed in FIG. 6, an 8 bytegap is needed per subframe of an ecoPN in a packing group. This gap“GAP” can no longer be compensated for further forwards on the line.Consequently, the 10 controller for each ecoPN principally needs between6*80 ns=560 ns and 10*80=800 ns (depending on the achievablePeerToPeerJitter) more receive time for the DFP frame than wouldactually be necessary without the gap.

In an implementation of the process discussed in FIG. 6, although theupstream neighbor of a device with subframe size below 15-18 bytes is“scheduled” later by this time, this time is, however, compensated forby further devices downstream which have a subframe size greater than15-18 Bytes (e.g., drives or modular devices, such as ET200S or ET200M),so that the frame at the IO controller is smaller overall.

Thus, while there have shown and described and pointed out fundamentalnovel features of the invention as applied to a preferred embodimentthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices illustrated, and intheir operation, may be made by those skilled in the art withoutdeparting from the spirit of the invention. For example, it is expresslyintended that all combinations of those elements and/or method stepswhich perform substantially the same function in substantially the sameway to achieve the same results are within the scope of the invention.Moreover, it should be recognized that structures and/or elements and/ormethod steps shown and/or described in connection with any disclosedform or embodiment of the invention may be incorporated in any otherdisclosed or described or suggested form or embodiment as a generalmatter of design choice. It is the intention, therefore, to be limitedonly as indicated by the scope of the claims appended hereto.

1. A method for data transmission in an automation system from a secondfield device via a first field device to a receiver, comprising:creating, at the first field device, a first data subframe comprisingprocess data of the first field device; receiving, at the first fielddevice, a second data frame from the second field device, the seconddata frame including a second data subframe with process data of thesecond field device; and correctly-timed sending, from the first fielddevice to the receiver, a first data frame comprising the first and thesecond data subframes by Dynamic Frame Packing; wherein, in an event ofthe second data subframe being unable to be appended directly to thefirst data subframe at a latest after the correctly-timed sending of thefirst data subframe, the sending of the first data frame is shifted by atime value; and wherein the time value is calculated so that immediatelyafter the sending of the first data subframe the second data subframe isappendable directly to the first data subframe.
 2. The method as claimedin claim 1, wherein the transmission time of the second data frame fromthe second field device to the first field device is included in thecalculation of the time value.
 3. The method as claimed in claim 2,wherein the transmission time comprises at least one of a time delay forsending the second data frame by the second field device, a time delayfor receipt of the second data frame by the first field device and atime delay corresponding to the cable delay time of the second dataframe between the second and first field devices.
 4. The method asclaimed in claim 1, wherein a lead time for performing the Dynamic FramePacking at the first field device is included in a transmission time. 5.The method as claimed in claim 1, wherein a time offset of local timesystems in the first and second field devices is included in thecalculation of the time value.
 6. The method as claimed in claim 1,wherein the first data frame is sent via a further field device to thereceiver; wherein the data transmission of each of the first, second andfurther field devices occurs in a time-synchronized manner; wherein foreach of the first, second and further field devices at least one of aglobal and an individual local send start offset (GlobalStartOffset) isa predetermined time delay; wherein a beginning of the data transmissionat the first and further field devices is delayed in relation to abeginning of the data transmission at the second and first fielddevices, respectively, at least by the individual local send startoffset; and wherein the sending of the first data frame by the firstfield device occurs at the earliest after the send start offset haselapsed.
 7. The method as claimed in claim 1, wherein, in an event of asum of at least one of a transmission duration of the second data framefrom the second to the first field device, a time offset of local timesystems in the first and second field devices and a lead time forperforming the Dynamic Frame Packing at the first field device isgreater than a time for sending an entire first data subframe, the timevalue becomes negative.
 8. The method as claimed in claim 1, wherein thetime value is calculated from a sum of at least one of a transmissionduration of the second data frame from the second field device to thefirst field device, a time offset of local time systems in the first andsecond field device, a lead time for performing the Dynamic FramePacking at the first field device and a time value by which the sendingof the second data frame at the second field device was delayed for thecorrectly-timed sending, minus a time for sending the entire first datasubframe.
 9. The method as claimed in claim 8, wherein, in an event ofthe calculated time value being smaller than the send start offset ofthe first field device, the send start offset of the first field deviceis used as the time value.
 10. A non-transitory computer program productencoded with a computer program executed by a processor that causes datatransmission in an automation system from a second field device via afirst field device to a receiver, the computer program comprising:program code instructions for creating, at the first field device, afirst data subframe comprising process data of the first field device;program code instructions for receiving, at the first field device, asecond data frame from the second field device, the second data frameincluding a second data subframe with process data of the second fielddevice; and program code instructions for correctly-timed sending, fromthe first field device to the receiver, a first data frame comprisingthe first and the second data subframes by Dynamic Frame Packing;wherein, in an event of the second data subframe being unable to beappended directly to the first data subframe at a latest after thecorrectly-timed sending of the first data subframe, the sending of thefirst data frame is shifted by a time value; and wherein the time valueis calculated so that immediately after the sending of the first datasubframe the second data subframe is appendable directly to the firstdata subframe.
 11. A first field device configured to receive data froma second field device and to send the data to a receiver, wherein thefirst field device is configured to: create a first data subframeincluding process data of the first field device; receive a second dataframe from the second field device, the second data frame including asecond data subframe with process data of the second field device; sendto the receiver correctly-timed by Dynamic Frame Packing a first dataframe including the first and the second data subframe; wherein, in anevent of the second data subframe being unable to be appended directlyto the first data subframe at a latest after the correctly-timed sendingof the first data subframe, the sending of the first data frame isshifted by a time value; and wherein the time value is calculated sothat immediately after the sending of the first data subframe the seconddata subframe is appendable directly to the first data subframe.